The present invention relates to a semiconductor device using a gallium nitride-based compound semiconductor and a manufacturing technique for the same and, more particularly, to a gallium nitride-based compound semiconductor laser, a method of manufacturing the same, and a method of manufacturing a gallium nitride-based compound semiconductor device.
In recent years, gallium nitride-based compound semiconductors such as GaN, InGaN, GaAlN, and InGaAlN have received a great deal of attention as materials for blue semiconductor lasers. Semiconductor lasers using these materials are designed as short-wavelength lasers and hence can focus beams to small diameters. Owing to this advantage, these lasers are expected to be used as light sources for high-density information processing such as processing in optical disks.
Various structures and manufacturing methods have been proposed for semiconductor lasers of this type. In either of these proposed techniques, a laser having satisfactory characteristics has not been obtained because a gallium nitride-based compound semiconductor layer is difficult to cause crystal growth. That is, even if a gallium nitride-based compound semiconductor layer is formed by crystal growth, a high-quality crystal cannot be obtained. Since the crystal quality is poor, carriers cannot be efficiently injected into the active layer. In addition, in a structure having a striped opening in a current blocking layer, the crystal quality of the regrown layer formed after etching for forming the striped opening deteriorates, resulting in voltage drops at the electrode contacts and the like.
To realize a highly reliable blue semiconductor laser which is used for optical disks and the like and operates at a low threshold and a low voltage, it is important to efficiently inject carriers into the active layer and suppress voltage drops at the electrode contacts and the like. However, with the prior techniques, these requirements cannot be satisfied.
According to a semiconductor laser using a gallium nitride-based compound semiconductor material, it is difficult to cause crystal growth of the material and to obtain a high-quality crystal layer. In addition, the crystal quality of the regrown layer formed after etching for forming a striped opening deteriorates. For these reasons, carriers cannot be efficiently injected into the active layer, and voltage drops occur at the electrode contacts and the like. That is, it is difficult to realize a highly reliable device which is used for optical disks and the like and operates at a low threshold and a low voltage.
That a gallium nitride-based compound semiconductor layer cannot be satisfactorily regrown after it is etched applies to various semiconductor devices using gallium nitride-based compound semiconductors as well as semiconductor lasers.
The present invention has been made in consideration of the above situations, and has as its object to provide a highly reliable gallium nitride-based compound semiconductor laser which allows efficient injection of carriers into the active layer, can suppress voltage drops at the electrode contacts and the like, is used for optical disks and the like, and operates at a low threshold and a low voltage, and a method of manufacturing the same.
It is another object of the present invention to provide a method of manufacturing a gallium nitride-based compound semiconductor device, in which a gallium nitride-based compound semiconductor can be properly regrown once it is etched, thereby contributing to an improvement in the characteristics of various semiconductor devices.
According to a first aspect of the present invention, there is provided a gallium nitride-based compound semiconductor laser comprising:
an active layer having a cyclic structure formed by cyclically stacking not less than two types of semiconductor layers;
first and second cladding layers of first and second conductivity types formed to sandwich the active layer so as to form a double-heterojunction structure;
first and second electrodes connected to the first and second cladding layers;
a current blocking layer formed between the second electrode and the second cladding layer and having a striped opening portion for constricting a current for the double-heterojunction structure; and
a current injection layer formed between the second electrode and the current blocking layer and in the opening portion and having an area larger than that of the opening portion,
wherein each of the first and second cladding layers, the current blocking layer, the current injection layer consists essentially of a material represented by the following composition formula:
InxGayAlzN
xe2x80x83where x+y+z=1, and 0xe2x89xa6x, y, zxe2x89xa61, and
when a thickness of the current blocking layer is represented by TA, and a distance between the current blocking layer and the active layer is represented by TB, a condition of TB less than TA is satisfied.
According to a second aspect of the present invention, there is provided a gallium nitride-based compound semiconductor laser comprising:
an active layer having a cyclic structure formed by cyclically stacking not less than two types of semiconductor layers;
first and second cladding layers of first and second conductivity types formed to sandwich the active layer so as to form a double-heterojunction structure;
first and second electrodes connected to the first and second cladding layers;
a current blocking layer formed between the second electrode and the second cladding layer and having a striped opening portion for constricting a current for the double-heterojunction structure; and
a current injection layer formed between the second electrode and the current blocking layer and in the opening portion and having an area larger than that of the opening portion,
wherein each of the first and second cladding layers, the current blocking layer, the current injection layer consists essentially of a material represented by the following composition formula:
InxGayAlzN
xe2x80x83where x+y+z=1, and 0xe2x89xa6x, y, zxe2x89xa61, and
when a thickness of the current blocking layer is represented by TA, and a thickness of the current injection layer excluding a portion in the opening portion is represented by TC, a condition of TC less than TA is satisfied.
Acdording to a third aspect of the present invention, there is provided a gallium nitride-based compound semiconductor laser comprising:
an active layer having a cyclic structure formed by cyclically stacking not less than two types of semiconductor layers;
first and second cladding layers of first and second conductivity types formed to sandwich the active layer so as to form a double-heterojunction structure;
first and second electrodes connected to the first and second cladding layers;
a current blocking layer formed between the second electrode and the second cladding layer and having a striped opening portion for constricting a current for the double-heterojunction structure; and
a current injection layer formed between the second electrode and the current blocking layer and in the opening portion and having an area larger than that of the opening portion,
wherein each of the first and second cladding layers, the current blocking layer, the current injection layer consists essentially of a material represented by the following composition formula:
InxGayAlzN
xe2x80x83where x+y+z=1, and 0xe2x89xa6x, y, zxe2x89xa61, and
when a distance between the current blocking layer and the active layer is represented by TB, and a thickness of the current injection layer excluding a portion in the opening portion is represented by TC, a condition of TB less than TC is satisfied.
According to a fourth aspect of the present invention, there is provided a gallium nitride-based compound semiconductor laser comprising:
an active layer having a cyclic structure formed by cyclically stacking not less than two types of semiconductor layers;
first and second cladding layers of first and second conductivity types formed to sandwich the active layer so as to form a double-heterojunction structure;
first and second electrodes connected to the first and second cladding layers;
a current blocking layer formed between the second electrode and the second cladding layer and having a striped opening portion for constricting a current for the double-heterojunction structure; and
a current injection layer formed between the second electrode and the current blocking layer and in the opening portion and having an area larger than that of the opening portion,
wherein each of the first and second cladding layers, the current blocking layer, the current injection layer consists essentially of a material represented by the following composition formula:
InxGayAlzN
xe2x80x83where x+y+z=1, and 0xe2x89xa6x, y, zxe2x89xa61, and
a portion, of the current injection layer, which is near the active layer has a band gap larger than a band gap corresponding to an emission wavelength of the active layer and a refractive index higher than that of the current blocking layer.
According to a fifth aspect of the present invention, there is provided a gallium nitride-based compound semiconductor laser comprising:
an active layer having a cyclic structure formed by cyclically stacking not less than two types of semiconductor layers;
first and second cladding layers of first and second conductivity types formed to sandwich the active layer so as to form a double-heterojunction structure;
first and second electrodes connected to the first and second cladding layers;
a current blocking layer formed between the second electrode and the second cladding layer and having a striped opening portion for constricting a current for the double-heterojunction structure; and
a current injection layer formed between the second electrode and the current blocking layer and in the opening portion and having an area larger than that of the opening portion,
wherein each of the first and second cladding layers, the current blocking layer, the current injection layer consists essentially of a material represented by the following composition formula:
InxGayAlzN
xe2x80x83where x+y+z=1, and 0xe2x89xa6x, y, zxe2x89xa61, and
the current blocking layer has a band gap larger than a band gap corresponding to an emission wavelength of the active layer, and a refractive index higher than that of a portion, of the current injection layer, which is near the active layer.
According to a sixth aspect of the present invention, there is provided a gallium nitride-based compound semiconductor laser comprising:
an active layer having a cyclic structure formed by cyclically stacking not less than two types of semiconductor layers;
first and second cladding layers of first and second conductivity types formed to sandwich the active layer so as to form a double-heterojunction structure;
first and second electrodes connected to the first and second cladding layers;
a current blocking layer formed between the second electrode and the second cladding layer and having a striped opening portion for constricting a current for the double-heterojunction structure; and
a current injection layer formed between the second electrode and the current blocking layer and in the opening portion and having an area larger than that of the opening portion,
wherein each of the first and second cladding layers, the current blocking layer, the current injection layer consists essentially of a material represented by the following composition formula:
InxGayAlzN
xe2x80x83where x+y+z=1, and 0xe2x89xa6x, y, zxe2x89xa61, and
a portion, of the current injection layer, which is near the active layer has a band gap larger than a band gap corresponding to an emission wavelength of the active layer, and the current blocking layer has a band gap smaller than the band gap corresponding to the emission wavelength of the active layer.
According to a seventh aspect of the present invention, there is provided a gallium nitride-based compound semiconductor laser comprising:
an active layer having a cyclic structure formed by cyclically stacking not less than two types of semiconductor layers;
first and second cladding layers of first and second conductivity types formed to sandwich the active layer so as to form a double-heterojunction structure;
first and second electrodes connected to the first and second cladding layers;
a current blocking layer formed between the second electrode and the second cladding layer and having a striped opening portion for constricting a current for the double-heterojunction structure; and
a current injection layer formed between the second electrode and the current blocking layer and in the opening portion and having an area larger than that of the opening portion,
wherein each of the first and second cladding layers, the current blocking layer, the current injection layer consists essentially of a material represented by the following composition formula:
InxGayAlzN
xe2x80x83where x+y+z=1, and 0xe2x89xa6x, y, zxe2x89xa61, and
a striped groove continuous with the opening portion is formed in an upper surface of the second cladding layer, and the current injection layer is formed to be in contact with the second cladding layer in the groove.
According to an eighth aspect of the present invention, there is provided a method of manufacturing a gallium nitride-based compound semiconductor laser, the semiconductor laser including:
an active layer;
first and second cladding layers of first and second conductivity types formed to sandwich the active layer so as to form a double-heterojunction structure;
first and second electrodes connected to the first and second cladding layers;
a current blocking layer formed between the second electrode and the second cladding layer and having a striped opening portion for constricting a current for the double-heterojunction structure; and
a current injection layer formed between the second electrode and the current blocking layer and in the opening portion and having an area larger than that of the opening portion,
wherein each of the first and second cladding layers, the current blocking layer, the current injection layer consists essentially of a material represented by the following composition formula:
InxGayAlzN
xe2x80x83where x+y+z=1, and 0xe2x89xa6x, y, zxe2x89xa61,
the method comprising the steps of:
forming a multilayer film having the double-heterojunction structure on a support substrate;
stacking the current blocking layer and a surface protective layer on the multilayer film;
forming the striped opening portion by partly etching the surface protective layer and the current blocking layer; and
forming the current injection layer in the opening portion and on the current blocking layer after removing the surface protective layer by leaving the surface protective layer in a vapor phase at a high temperature to re-evaporate crystals.
According to a ninth aspect of the present invention, there is provided a method of manufacturing a gallium nitride-based compound semiconductor laser, the semiconductor laser including:
an active layer;
first and second cladding layers of first and second conductivity types formed to sandwich the active slayer so as to form a double-heterojunction structure;
first and second electrodes connected to the first and second cladding layers;
a current blocking layer formed between the second electrode and the second cladding layer and having a striped opening portion for constricting a current for the double-heterojunction structure; and
a current injection layer formed between the second electrode and the current blocking layer and in the opening portion and having an area larger than that of the opening portion,
wherein each of the first and second cladding layers, the current blocking layer, the current injection layer consists essentially of a material represented by the following composition formula:
InxGayAlzN
xe2x80x83where x+y+z=1, and 0xe2x89xa6x, y, zxe2x89xa61,
the method comprising the steps of:
forming a multilayer film having the double-heterojunction structure on a support substrate;
forming the current blocking layer on the multilayer film;
forming the striped opening portion by partly etching the current blocking layer; and
forming the current injection layer in the opening portion and on the current blocking layer after dipping the support substrate, the multilayer film, and the current blocking layer in a solution of HNO3:HCl=3:1 heated to not less than 100xc2x0 C.
According to a tenth aspect of the present invention, there is provided a method of manufacturing a gallium nitride-based compound semiconductor laser, the semiconductor laser including:
an active layer;
first and second cladding layers of first and second conductivity types formed to sandwich the active layer so as to form a double-heterojunction structure;
first and second electrodes connected to the first and second cladding layers;
a current blocking layer formed between the second electrode and the second cladding layer and having a striped opening portion for constricting a current for the double-heterojunction structure; and
a current injection layer formed between the second electrode and the current blocking layer and in the opening portion and having an area larger than that of the opening portion,
wherein each of the first and second cladding layers, the current blocking layer, the current injection layer consists essentially of a material represented by the following composition formula:
InxGayAlzN
xe2x80x83where x+y+z=1, and 0xe2x89xa6x, y, zxe2x89xa61,
the method comprising the steps of:
forming a multilayer film having the double-heterojunction structure on a support substrate;
forming the current injection layer on the multilayer film; and
forming the current blocking layer having the striped opening portion by introducing an impurity into a portion of a selected layer formed of at least one of the cladding layer and the current injection layer.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.